In a communication device, it is typical that functionality for the device is embodied into several modules. For example, in a communication switch, modules include line cards, switching fabrics and control systems. In turn, each module may itself comprise several submodules. This may continue iteratively for sub-modules. As circuit densities increase on integrated circuits, such circuits are commonly developed as a series of interconnected modules. Signals are communicated between modules through connections between modules. Due to the increasing densities and operating frequencies of the integrated circuits, signals carried on the connections are prone to contain errors.
For the modules of a communication device, a messaging system is commonly used. Therein, each module may generate a message having administrative and payload information. The administrative information may include the intended destination of the message, the size of the message and error management information. The message is transmitted over a communications link from the source module to the destination module.
Frequently, messages and data packets processed by a communication device must be maintained in some order. A known method of tracking data packets is to associate a sequence number with each packet. When a communication device receives a packet, its modules process the packet internally and eventually the device transmits the packet to another communication device. In order to facilitate the internal processing of the packets by the communication device, frequently a separate message is associated with each packet. The messages are transmitted between the modules in conjunction with the processing of the related packets by the modules. It will be appreciated that as the messages contain administrative information regarding their packets, the modules which process the messages may not necessarily be the same modules which process the packets. As an example, in processing a packet, one module may be responsible for determining an action for a packet (e.g. queue, discard, modify etc) and another module may perform the action.
When transmitting messages, a module which transmits a message needs to know whether a transmitted message was received by the destination module. A failure in the communications link or an error in the message itself may cause the message to be lost. As a solution, prior art messaging systems track messages by having a module which receives a message generate and transmit an acknowledgement (ACK) message back to the transmitting module. When the transmitting module eventually receives the ACK message, it knows that the transmitted message was received.
There are deficiencies with the transmit/ACK message scheme. For example, if a failure occurs when transmitting the initial message by the transmitting module, the intended receiving module will not receive the message and will not generate a replying ACK message. In this situation the transmitting module will be waiting, perhaps indefinitely, for the ACK message. If a failure occurs when transmitting the ACK message by the receiving module, the originating transmitting module will not receive the ACK message and will not have knowledge that the original message was, in fact, successfully received by the receiving module. These failures are not equivalent in that the receiving module is in a different state either having seen the original message or not. There are many situations in which the loss of a message may result in a persistent impairment to the system's operation. Given that the transmit/ACK messaging scheme does not differentiate between these failure mechanisms, the only corrective action to take may be to reset the system. Evidently, this will disrupt the operation of the system.
There is a need for a system and method for providing improved messaging scheme between two modules wherein individual failures of transmission of messages between the modules can be detected and accommodated.